Velocity of Transmission of Disturbances through Sea-water. 533 



vations lay in the fact that at first we could see no reason why,, if the 

 gauges worked at all, they should give results which differed. We 

 could not see that the introduction of water, for instance, between the 

 rubber faces ought to make the time- constant of the gauge greater — 

 and yet at first we could only explain the abnormal low velocities by 

 supposing that the time-constants were enormously increased at the 

 further stations. A fortunate observation, however, on September 

 13th, put us on the right track. We had noticed on several occasions 

 that the first mark made by the scriber on the smoked plates was not 

 nearly so strong as the second or third. These marks did not depend 

 in any way on the natural period of vibration of the scriber, for that 

 was far too short, and hence we put them down to the passage up and 

 down the air-tube of the gauge of pressure waves. It was not yet 

 clear, however, why it was that sometimes the second and third marks 

 of the scriber were stronger than the first. 



The strength or distinctness of these marks clearly depends on the 

 conductivity and duration of the contact effected in the gauge head ; 

 and this must depend on the increase of pressure and on its duration 

 in the water in the neighbourhood of the rubber disks. Now it is 

 probable that several waves and not a single wave of compression 

 start from an explosive centre. First there is the sudden expansion 

 by the explosion, then the cooling or escape of the gases and the con- 

 sequent falling together of the water before the disturbance sub- 

 sides. 



A glance at the map will show that our gauges were so situated 

 with respect to the sheer stone wall of the quay that interference 

 might well be looked for. Supposing then that interference takes place 

 at the further gauge between the direct and reflected waves ; it is 

 clear that the first pressure to which the gauge is exposed is not 

 necessarily the strongest, and equally clear that the duration of the 

 interference pressure may be longer, but cannot very well be shorter 

 than that of the pressure first arriving. It may well happen, there- 

 fore, that the second or third marking on the plate is the most 

 distinct, or, even if the sensitiveness of the gauge is low, the first 

 mark might be suppressed entirely, and the second appear alone in 

 its stead. Now, the distance from the first to the second marking 

 on the plate was pretty constant so long as the conditions of the 

 experiment remained the same, and, strangely enough, as it then 

 seemed to us in the case of the abnormal results referred to, there 

 was also a certain agreement. It seemed as if the velocity was 

 either a good deal greater than the velocity of sound — or else a good 

 deal less ; the abnormal velocities were consistent with themselves in 

 a rough way. On September 13th, however, we found out the reason 

 — on that day we got a record in which the first motion of the 

 scriber (giving a normal time) was all but too small for detection— 



